I had this same problem with my 78. If your have vacuum going to the control panel and the control panel is plugged in correctly to the vacuum hoses, it is either a hose problem or the actuator motor used to open that particular door is no good.
I would get some vacumm tubing and replace that first since it is cheapest. You will have to take apart the dash by removing the radio, the guage cluster and a few other parts. Make sure not to confuse the hoses as each vacuum motor must coincide with a numbered "port" on the control panel. This is a pretty big job, so don't start if you don't have a full day to jack with it. If you replace the hose and it still won't work and the control assembly is hooked up to the correct hoses, try changing the actuator motor. If you need to know which hose goes where on the operating assembly, let me know and I'll put it up. It is nice once it works correctly. I had to replace everything though, including the contrrol assembly.

Hi! jhr1992 You need to get the vacuum system checked. The A/C is controlled by engine vacuum. Make sure the vacuum hoses are connected under the hood expecialy the one feeding through the fire wall to the AC Control. You should not hear any vacuum leak sound from under the dash. Sometimes you need to pull the control and just do some repair or replacement. InMrFixIt

Tell us some more! Your answer needs to include more details to help people.You can't post answers that contain an email address.Please enter a valid email address.The email address entered is already associated to an account.Login to postPlease use English characters only.

You most likely have a vacuum leak not allowing the vacuum operated vent/duct inside your instrument panel to operate. Check all of your rubber vacuum hoses in the engine compartment for leaks, cracks or breaks .

the a/c control is electric for temp control and vacuum for mode control when a leak occurs it defaults to defrost floor .look for a leak or broken vacuum line first then control panel then mode actuator.

this is because
1.there is some blockage in the line comin up and to sides.
2.the selector switch is not properly functioning. the flaps got stuck and it may allows only air to pass through the floor vents.

Your last guess is probably the right one.
The deflectors/directors are vacuum controlled and you may have vacuum line either broken or disconnected; these aren't fun to find.
I would start under the hood and make sure the vacuum reservoir is intact (have seen them cracked) and then start tracing hoses from there.

I don't know about your 'vette specifically, but many cars use vacuum actuators to control where which vents receive the blower's air. This symptom suggests you have a broken or disconnected vacuum hose.

Its just a vent damper plate not operating, it's like a gate which controls the flow of air,,usually they are operated by vacuum, or cables.Service center time, unless you are handy with your own repairs.